Physical strength is more of an advantage on the ground than it is in standing technique. Ground technique also develops faster than throwing does. Going to newaza is often a good strategy when confronting a more advanced opponent of equal size and strength.
When I was a teenager, I was tall and skinny--not much upper body strength. Like you, I could throw well, but on the ground I could get easily overpowered. My strategy used to be, if I was taken to the ground after a missed throw, I would quickly scramble out and get back to my feet again. It allowed me to defeat much stronger, larger guys, but I hated that I didn't have the technique to win on the ground.
Over time, I realized that the principles of standing judo work on the ground as well. I had never been taught it in that way; I had to figure it out for myself. Basically, to throw someone, you need to get him off balance, control the space, and apply an effective technique at the right moment. The same works on the ground. You have to break uke's balance to offset a strength advantage. You have to keep adequate space in which to maneuver or uke can overpower you.
In standing technique, we practice combinations--two or three throws in sequence or a throw into a hold. Consider practicing ground combinations for the situation in which you get dragged down by a strong uke or miss a throw. For example, practice quickly getting into guard as a temporary defensive position and immediately transition it to an arm bar or choke. Don't give uke the time to do a guard pass and pin you. When you are going down, you already have the sequence rehearsed. Then you can react quickly before uke can begin to apply his technique.
Another tip is to learn to use your legs effectively. I learned this as a wrestler. My legs are much stronger than any opponent's arms. You can use butterfly guard as an offensive technique to sweep an aggressor from the top position and either get back to your feet or go on the attack on the ground. There are a number of leg techniques like sankaku jime, ashi gatame, hiza gatame, and others that will allow you to fend off someone stronger.
Another thing you can do is observe other players who are successful on the ground, especially if they are about your size and weight, and watch what they do. Pay attention to how they control balance, space, and what techniques they apply. If you are conscious of these things when you watch, you'll be able to emulate it in randori.